CSV is a Comma Separated Variable file. These are simple text files with data in lines and text separated by commas. They are used as ways of storing data so it can be imported from one application to another. A CSV file could be created in Excel and then opened in a database such as Access. Each line would be a record and where each comma would be it would start a new field. Coming from Excel, each comma would have been created from the division of each cell, so data could be taken from Access, put into a CSV format and transferred to Excel and many applications. So CSV files are not limited to Excel.
xlsx is the main extension used for Excel 2007.
For a standard Excel document, prior to Excel 2007, it is xls and xlsx is used in 2007. There are other kinds of files created by Excel, like templates which can be xlt or macros which can be xlm and xla for Add-ins, and there are other extenions. For 2007, you add x to the end of them.
The standard extension for Excel workbooks is xls for versions up to 2003. Excel 2007 uses xlsx as its extension for workbooks.
For workbooks in Excel 2003, the extension .xls is used.
Excel 2013 uses a .xlsx extension.
You are referring to the the extension. For Powerpoint it is PPT and XLS is used by Excel.
.mdb is the file extension used by Microsoft Office's Access program and it marks a file that is a database.
The extension is used to identify the file type and enable it to be opened by the right software. By having extensions we can also give files that are related the same name in different applications. So you could have a file for finances in a Word document called Finances.doc and one in Excel called Finances.xls too. You can also estimate the versions of the software that created the file. Up to Excel 2003 .xls was used and since Excel 2007 .xlsx has been used.
XLS is the main extension name used in Excel.
The file extension. .doc, .xml, .exe, etc. Only applicable to Windows files. Linux does not use file extension and MAC uses file types and creator codes.
The file extension for PHP is .php
.XLS files are standard Microsoft Excel spreadsheet files. .XLT extension is used for the Excel template files. - Neeraj Sharma